Clinical and necropsy observations are described in 46 patients who had one or more cardiac valves replaced with a tilting disc prosthesis of the Bjork-Shiley type and these observations are compared to those made earlier in patients with caged-ball, caged-disc and trileaflet prostheses. Although it is prone to clot and therefore requires long term anticoagulation, the tilting disc (Bjork-Shiley) prosthesis is not intrinsically stenotic or significantly incompetent, it is only minimally traumatic to erythrocytes, and, at this point in time, there is no evidence of prosthetic degeneration or variance.